With the recent rapid advancement of electronic equipment in terms of function, weight and size reduction, there has been a demand for high-density mounting of electronic components. To achieve these objectives, it is important to increase the density of PWB. A so-called build-up system has been proposed as one approach for increasing the PWB density. The build-up system is characterized in that interlayer connection is achieved by forming fine via holes through a photosensitive interlayer dielectric instead of drilling conventional through-holes.
A specific example of a build-up system which makes use of a photosensitive interlayer dielectric is disclosed in JP-A-4-148590 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In this method, a photosensitive insulating resin layer is provided on a first circuit pattern. After forming via holes by photolithography, the resin layer is subjected to chemical surface roughening. The chemical surface roughening enhances adhesion of the resin layer to an electroless copper deposit and also to an electro copper deposit overlaid thereon. The chemical treatment provides the resin surface with fine unevenness to thereby achieve improved adhesion through an anchoring effect.
However, the adhesion achieved by providing surface unevenness (surface roughening treatment) in JP-A-4-148590 is still insufficient, leaving room for further improvement. Moreover, chromic acid, etc. which is used in the chemical surface roughening, is unfavorable in consideration of safety and environmental conservation.
JP-A-63-126297 discloses a method comprising hardening a photosensitive insulating resin having dispersed therein fine particles soluble in an acid or an oxidizing agent and dissolving the dispersed particles with a strong acid or a strong oxidizing agent, such as chromium (VI) oxide. This forms unevenness on the resin surface, to thereby improve adhesion to a metal deposit.
However, the treating agents used in the technique of JP-A-63-126297 are also problematical for safety and environmental consideration. In addition, when the photosensitive resin layer is provided on an insulating substrate by a laminating method (heat bonding or press bonding), which is simpler than a printing or coating technique and is relatively free from defects, defects such as bubble entrapment on the surface of the insulating substrate tend to occur because of the fine particles contained in the photosensitive insulating resin. Thus, this technique does not provide a practically useful product.